1. Field of the Invention
The present invention relates to a method for improving the internal center segregation and center porosity of a continuously cast strand, particularly a slab.
2. Description of the Related Art
Techniques for producing continuously cast strands, for example, slabs booms, and billets, etc. are disclosed in three publications, i.e., Japanese Unexamined Patent Publication (Kokai) Nos. 62-89555 and 62-259647 and Japanese Examined Patent Publication (Kokoku) No. 63-5904. These references disclose a method and a device for preventing the generation of internal center segregation and center porosity. These references disclose the use of two sets of opposing inner and outer walking bars to compress the surface portions of the slab having unsolidified molten metal in the interior. The top face of the lower bars is aligned with the bottom side of the cast strand slab so as to coincide with the lower side pass line of the continuous casting machine, and the top face of the lower bars contact the bottom side of the cast strand. The bottom face of the upper bars contact the top side of the cast strand to provide a selected compression gradient or plane reduction taper. The inclination or reduction ratio given the top bars is based on the compression gradient or reduction taper, converted to unit length, needed to prevent solidification shrinkage motion or flow, thermal shrinkage, and bulging motion or flow from causing internal center segregation and center porosity. This is determined in accordance with the amount of solidification shrinkage and the amount of thermal shrinkage of the solidified shell. The solidified shell of the unsolidified end portion of the cast strand is alternately compressed or plane reduced by each set of walking bars across the width of the cast strand. As a result, motion of impurity-enriched molten steel toward the unsolidified end portion of the cast strand, and solidification of the impurity-enriched molten steel at the unsolidified end portion are sought to be prevented. The expansion of the unsolidified end portion and gap formation are also sought to be prevented. The above-mentioned device and method do indeed, at times, alleviate the problems of the end center segregation and center porosity generated at a cast strand slab width center portion. The improvement is not uniformly achieved and the quality of the produced material may vary in the width direction.
The present inventors found by experiments that the reasons for such non-uniform quality in the width direction is an imbalance in compression (plane reduction) between the walking bars.
The walking bars are designed to give uniform compression. However, imbalances in actual practice are mainly generated due to the following reasons.
1) Temperature deviation across the width direction of the cast slab due, e.g., to non-uniform cooling.
2) There can be different degrees of solidification across the width of the portion of the slab being compressed. E.g., the degree of solidification at the center may be significantly different than the degree of solidification at the edges as one passes across the width of the portion of the slab being compressed. The walking bars at the edge portions may be pressing against a completely solidified thickness.
3) Non-uniform shape of the strand slab due to bulging and other irregularities caused by the rolls located in front of the walking bars can have an influence.
4) The present inventors found that center segregation and center porosity are improved by the following: balance of the compression gradients (reduction tapers) of the top walking bars in the longitudinal direction of the cast strand slab; balanced compression of the upper surfaces of the bottom walking bars; control of the deviation of the actual passline of the cast strand slab in comparison to the passline of the continuous casting machine; balance between the reaction forces derived from the top and bottom slab surfaces under compression. In the present specification, compression has the same meaning as plane reduction.